@xml:id(optional)Regularizes the naming of an element and thus facilitates building links between it
and other resources. Each id attribute within a document must have a unique
value.
Value of datatype
ID.
att.id

@xml:lang(optional)Identifies the language of the element's content. The values for this attribute are
language 'tags' as defined in BCP 47. All language tags that make use of private use
sub-tags must be documented in a corresponding language element in the MEI header
whose
id attribute is the same as the language tag's value.
Value of datatype
language.
att.lang